Chaffoteaux boiler MINIMA blowing apart flue pipes?

HI all. I have problem with Chaff MINIMA boiler.
Ariston have engineers but not in my area and will not travel. So need help! When boiler fires up the flue pipes blow apart at joints. I can only think this is due to late combustion of the gas? Any one come across this before and know the fix so I can get an engineer in to do work.
Wayne, June 2013


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